Liver allotransplantation after extracorporeal hepatic support with transgenic (hCD55/hCD59) porcine livers: clinical results and lack of pig-to-human transmission of the porcine endogenous retrovirus

History of xenotransplantation

The present historical review reports the clinical experiences of transplantations from animal to human. The first transplantation attempts were made without any knowledge of the species barrier. The pioneers of xenotransplantation realized xenotransfusions as early as the 16th century, then cell and tissue xenotransplantations in the 19th century. At the beginning of the 20th century, xenotransplantation of testicles became the latest craze. At the same time, and later in the 1960s, organ xenotransplantations were attempted, with disappointing results. Mathieu Jaboulay, Serge Voronoff, Keith Reemtsma, James Hardy, Denton Cooley, Thomas Starzl, Christiaan Barnard and Leonard Bailey were among the pionneers of xenotransplantation. Recent trials concerned above all tissue and cell xenotransplantations. Nowadays, with encapsulation, transgenesis, and cloning, great advances have been made for controlling xenograft rejection, but ethical questions linked to the risk of infections have become a major pre-occupation within the scientific community and the general population.

MELD Scores Do Not Predict Patient Morbidity While on the Liver Transplant Waiting List

The goals of this study were to assess waitlist morbidity in terms of the frequency of health care services utilized by patients while on the liver transplant (LTX) waiting list and to determine whether that utilization can be predicted by the Model for End-Stage Liver Disease (MELD). Sixty-three noncomatose subjects were followed from waitlist placement until death, change in status, LTX, or study discontinuance. Health care events included doctor/clinic visits, labs, outpatient/inpatient tests and procedures, and hospital/intensive care unit days. Listing MELD scores and LTX MELD scores were examined against the number of health care event occurrences within 60 days of listing and 60 days of LTX, respectively, as were changes in MELD scores between listing and LTX and differences in the number of occurrences between the two time points. The only significant correlations noted were between LTX MELD scores and number of hospital days near LTX (r = .360, P = .046) and between LTX MELD scores and the sum total number of occurrences near LTX (r = .370, P = .044). These results suggest that MELD scores do not appear to predict morbidity in terms of health care utilization in patients awaiting LTX. Developing a system capable of predicting waitlist morbidity may lead to the implementation of medical interventions aimed at circumventing foreseeable complications and/or crises in patients awaiting LTX.

Absence of PERV infection in baboons after transgenic porcine liver perfusion

BACKGROUND

Xenotransplantation offers great promise to supplement the shortage of human organs available for transplant, but cross-species infection is a substantial concern. Porcine endogenous retrovirus (PERV), in particular, is thought to pose a risk as a potential pathogen to humans. We evaluated whether PERV is capable of infecting nonhuman primates in vivo after extracorporeal porcine liver perfusion (ECLP).

METHODS

Livers were harvested from six human decay-accelerating factor (h-DAF) transgenic piglets and perfused with fresh baboon blood via the portal vein and the hepatic artery. Six healthy baboons underwent direct cross-circulation with the ECLP for 13 to 24 h without immunosuppression. Peripheral blood and bone marrow of baboons were sampled periodically until the baboons were euthanized for the examination of various organ tissue samples. Genomic DNA was extracted from those samples and tested for PERV and pig-specific centromeric DNA sequences by quantitative PCR. Validation showed that the assay could detect one copy of PERV in a background of 150,000 baboon cells, and it was quantitative over a range from 10 to 10(6) copies of PERV.

RESULTS

PERV sequences were detected in a high number (4.4 x 10(3)-1.6 x 10(4)/1 microg) in peripheral leukocyte DNA during the initial phases of ECLP, but they disappeared within 1 week. Bone marrow DNA contained PERV sequences longer than peripheral blood, but PERV signals became negative within 1 month. No PERV DNA relapse was seen over the course of this study. Pig-specific centromeric sequences were also detected in the same manner. At 6 months or 1 year after ECLP, no PERV or pig-specific centromeric sequences were detected in the genomic DNA obtained from the following organs: skin, lymph nodes, spleen, liver, pancreas, kidney, heart, and lung.

CONCLUSIONS

ECLP did not result in PERV infection or pig-cell microchimerism in baboons.

Les xénogreffes finiront-elles par être acceptées ?

Transplantation represents a major advance in modern medicine with a major impact on the interactions between individuals and society. The numbers of patients undergoing organ transplantation increased steadily over the years and around 250,000 individuals are living nowadays in Europe with a transplanted organ. On the other hand, the numbers of cadaveric (brain-dead) donors used for organ transplantation remains stable, at around 5,000 each year, and the numbers of transplantation from living donors only slowly increase in Europe. Therefore, a gap is growing between the numbers of patients in need of a transplant and the numbers of organs available for transplantation. About 45,000 patients are currently on renal transplant waiting lists in Europe and, depending on the countries considered, 15 to 30 % of candidates for liver or heart transplantation die before a life-saving transplant becomes available to them. There is therefore an urgent need to implement innovative research and to take full advantage of recent biotechnological advances to explore new avenues in xenotransplantation, and to simultaneously address the ethical, societal and public health issues related to organ replacement. Much progresses have been accomplished in the understanding of xenograft rejection processes that include hyperacute, acute vascular and cellular rejection mechanisms. Strategies to promote xenograft survival that are currently under evaluation include genetic engineering of donor pigs, adapted immunosuppressive treatments and tolerance induction. Also, the psychological acceptance has been evaluated.

Evidence and consequence of porcine endogenous retrovirus recombination

The genetic nature and biological effects of recombination between porcine endogenous retroviruses (PERV) were studied. An infectious molecular clone was generated from a high-titer, human-tropic PERV isolate, PERV-A 14/220 (B. A. Oldmixon, et al. J. Virol. 76:3045-3048, 2002; T. A. Ericsson et al. Proc. Natl. Acad. Sci. USA 100:6759-6764, 2003). To analyze this sequence and 15 available full-length PERV nucleotide sequences, we developed a sequence comparison program, LOHA(TM) to calculate local sequence homology between two sequences. This analysis determined that PERV-A 14/220 arose by homologous recombination of a PERV-C genome replacing an 850-bp region around the pol-env junction with that of a PERV-A sequence. This 850-bp PERV-A sequence encompasses the env receptor binding domain, thereby conferring a wide host range including human cells. In addition, we determined that multiple regions derived from PERV-C are responsible for the increased infectious titer of PERV-A 14/220. Thus, a single recombination event may be a fast and effective way to generate high-titer, potentially harmful PERV. Further, local homology and phylogenetic analyses between 16 full-length sequences revealed evidence for other recombination events in the past that give rise to other PERV genomes that possess the PERV-A, but not the PERV-B, env gene. These results indicate that PERV-A env is more prone to recombination with heterogeneous backbone genomes than PERV-B env. Such recombination events that generate more active PERV-A appear to occur in pigs rather frequently, which increases the potential risk of zoonotic PERV transmission. In this context, pigs lacking non-human-tropic PERV-C would be more suitable as donor animals for clinical xenotransplantation.

Extracorporeal liver support: waiting for the deciding vote

Because acute liver cell failure is associated with an exceedingly high mortality, liver support has been proposed since the 1950s to improve patient outcome. Early devices, including hemodialysis, hemofiltration, exchange transfusion, plasmapheresis, hemoperfusion, plasma and cross-hemodialysis or cross-circulation, appeared inefficient. Meanwhile, documented results of extracorporeal liver perfusion (ECLP) suggested its superiority over conventional treatment. These devices were abandoned with the development of liver transplantation (LT), which allowed a better outcome and longer survival rate. In the present day, the fact that patients die while waiting for LT because of organ shortage led to a renewed interest in liver support as bridge to LT or regeneration. These devices can be classified according to the presence or lack of hepatocytes, whereas biologic devices refers to the presence of cells or other organic and biochemical component. The absence of individual success of early models led to the development of combined hepatocyte free devices, or artificial liver, which are based upon the hemodiabsorption principle (Biologic-DT) or on the "albumin bound toxin hypothesis" (Molecular Adsorbents Recirculating System) with encouraging results. Meanwhile, hepatocyte based bioartificial liver devices (BLD) were conceived for a global "metabolic support." BLD were developed with the use of human hepatoma cell line (C3A) or primary or cryopreserved porcine hepatocytes. Preliminary experience gave promising results bridging patients to LT. Based upon the same principle of global hepatocyte metabolic support, ECLP regained interest, particularly with the development of transgenic pigs. Several concerns were raised about these devices. Artificial livers lacked any metabolic synthetic activity, the use of human liver for ECLP seems hardly acceptable because of organ shortage, and the accepted use of borderline livers for transplantation is pending trials for the use of xenogenic livers. For BLD, the concerns were the low hepatocyte mass, the absence of accessory liver cells, and the potential risk of seeding tumor cells into patient with the use of human hepatoma cell line. The use of porcine hepatocytes (BLD or ECLP) raised physiologic and immunologic concerns and particularly the fear of a possible transfer of porcine viral material. Although recent studies clearly demonstrate clinical improvement of patients with the use of recently developed liver support devices, most of reported prospective, controlled, or randomized trials had a small number of patients. To give the deciding vote and avoid previous pitfalls, trials need to be developed with a larger number of patients based upon statistically significant models with the following characteristics: 1) comprehensive understanding of the acute liver cell failure mechanisms, 2) world wide classification of conditions that require liver support, and 3) a clear definition of treatment success pending patients to LT or recovery without transplantation. There has not yet been conclusive evidence to support the benefits of extracorporeal liver support. We are still waiting for the deciding vote.

De novo liver tissue formation in rats using a novel collagen-polypropylene scaffold

In experimental and clinical settings hepatocyte transplantation has provided limited benefit to patients with chronic liver disease because the transplanted hepatocytes were short-lived and were merely maintained for a brief period within the body. Except for whole-liver transplantation, creation of de novo liver tissue is necessary to treat this condition on a long-term basis. The aim of this study was to facilitate the formation of new tissue by actual self-regeneration, rather than by compensatory hypertrophy, or scar formation, with our collagen-polypropylene composite scaffold. Collagen-polypropylene composite scaffolds, not containing hepatocytes, were implanted into the median liver lobe and the dynamics of new liver tissue formation was analyzed immunohistochemically over a 6-month period. Control scaffolds consisted of polypropylene scaffolds without collagen matrix. The control scaffold implants remained hollow throughout the study period and became encapsulated with a hard connective tissue capsule 1 week after implantation. In contrast, the collagen-polypropylene composite scaffold was filled with regenerating tissue structures 3 weeks after implantation. At this time, the predominant cell type within the scaffold was sesmin-positive stellate cells. A week earlier, oval cells were identified using monoclonal antibody staining (OV-6). Subsequently, these cells differentiated into alpha-fetoprotein-positive immature hepatocytes. After 6 months, mature liver tissue, juxtaposed with bile ducts and blood vessels, was seen within the polypropylene scaffolds. We report the first evidence of de novo formation of liver tissue within a polypropylene scaffold, following implantation in the liver. This scaffold may play a role in treating chronic liver diseases requiring organ replacement therapy.

Alloantibody and xenoantibody cross-reactivity in transplantation

The recent availability of pigs homozygous for alpha1,3-galactosyltransferase gene knockout, and improved immunosuppressive regimens that prevent an elicited antibody response, are expected to contribute to significantly increased survival of pig organs transplanted into primates, bringing clinical trials of xenotransplantation closer. Patients highly sensitized to human leukocyte antigens, who may be precluded from obtaining a human donor organ, would be one group that might benefit from xenotransplantation. However, there have been few studies on whether there is cross-reactivity of anti-human leukocyte antigen antibodies with pig antigens. What data there are suggest that such cross-reactivity exists and that this may be detrimental to the outcome after transplantation of a pig organ. Neither is it known whether sensitization after a pig xenograft would preclude subsequent allotransplantation, although the data available suggest that this will not be the case. Further investigation on allo- and xenoantibody cross-reactivity is required.

Absence of humoral and cellular alloreactivity in baboons sensitized to pig antigens

AIM

to study whether sensitization to pig antigens results in humoral and/or cellular sensitization to alloantigens in baboons, and thus increases the risks of organ allotransplantation after xenotransplantation. Serum from baboons that were naive (n = 4), sensitized to Gal alpha 1,3Gal (Gal) antigens (n = 2), or sensitized to Gal + non-Gal pig antigens (n = 2) were tested by flow cytometry for the presence of immunoglobulin G (IgG) and IgM antibodies that bind to pig or baboon peripheral blood mononuclear cells (PBMC). Two allosensitized baboons were used as positive controls. The same 10 sera were tested in a complement-mediated cytotoxicity assay to detect cytotoxic antibodies against pig, allo and self-PBMC. The T-cell responses of the same baboons to allogeneic and pig PBMC stimulators in mixed lymphocyte reaction (MLR) were studied. All baboon sera contained cytotoxic antibodies that bound to pig PBMC. Binding and cytotoxicity were higher in xenosensitized baboons, particularly in those sensitized to Gal + non-Gal antigens (P < 0.001). None of the naive or xenosensitized baboon sera bound to baboon PBMC. Serum from allosensitized baboons showed anti-baboon IgG and IgM binding, but there was no increase in binding to pig PBMC or in cytotoxicity to pig cells. The MLR response to pig stimulators in baboons sensitized to non-Gal pig antigens was greater than that of naive or Gal-sensitized baboons (P < 0.001), but there was no increase in the response to baboon cells. In baboons, no in vitro evidence that a previous pig xenograft might endanger the outcome of a subsequent allograft was documented.

Xenografts are an achievable breakthrough

The objective of this communication is to show that pig-to-human organ transplantation could be feasible through genetic engineering. By introducing into donor pigs several different tolerance promoting genetic modifications there can be a synergistic effect to produce extended tolerance for xenografted organs in human recipients. Nuclear-transfer cloning allows production of pigs with knockout mutations in the galactose-alpha-1,3-galactosyl transferase gene, in principle eliminating hyperacute rejection. Once hyperacute rejection is circumvented, long-term tolerance of xenografted organs should be possible through a combination of transgenic immunomodulating molecule, bone marrow chimerism and short to intermediate term use of immunosuppressive drugs. If immunomodulating transgenes are deleterious during pig development, inducible cre-recombinase excision of stop codons provides a means to delay expression of such transgenes until after transplantation. Zoonotic diseases can be circumvented via pathogen-free colonies and additional knockout mutations to disable porcine endogenous retrovirus and prion disease. Thus, there is now a technical and theoretical framework for serious efforts at cross-species transplantation.

Xenotransplantation and risks of zoonotic infections

The shortage of human organs and tissues for transplantation and the advances in immunology of rejection and in genetic engineering have renewed interest in xenotransplantation--the transplantation of animal organs, tissues or cells to humans. Clinical trials have involved the use of non-human primate, porcine, and bovine cells/tissues/organs. In recent years, research has focused mainly on pigs as donors (especially, pigs genetically engineered to carry some human genes). One of the major concerns in xenotransplantation is the risk of transmission of animal pathogens, particularly viruses, to recipients and the possible adaptation of such pathogens for human-to-human transmission. Porcine endogenous retroviruses (PERVs) have been of special concern because of their ability to infect human cells and because, at present, they cannot be removed from the source animal's genome. To date, retrospective studies of humans exposed to live porcine cells/tissues have not found evidence of infection with PERV but more extensive research is needed. This article reviews infectious disease risks associated with xenotransplantation, some measures for minimizing that risk, and microbiological diagnostic methods that may be used in the follow-up of xenotransplant recipients.

Tissue engineering and cell based therapies, from the bench to the clinic: the potential to replace, repair and regenerate

The field of Regenerative Biology as it applies to Regenerative Medicine is an increasingly expanding area of research with hopes of providing therapeutic treatments for diseases and/or injuries that conventional medicines and even new biologic drug therapies cannot effectively treat. Extensive research in the area of Regenerative Medicine is focused on the development of cells, tissues and organs for the purpose of restoring function through transplantation. The general belief is that replacement, repair and restoration of function is best accomplished by cells, tissues or organs that can perform the appropriate physiologic/metabolic duties better than any mechanical device, recombinant protein therapeutic or chemical compound. Several strategies are currently being investigated and include, cell therapies derived from autologous primary cell isolates, cell therapies derived from established cell lines, cell therapies derived from a variety of stem cells, including bone marrow/mesenchymal stem cells, cord blood stem cells, embryonic stem cells, as well as cells tissues and organs from genetically modified animals. This mini-review is not meant to be exhaustive, but aims to highlight clinical applications for the four areas of research listed above and will address a few key advances and a few of the hurdles yet to be overcome as the technology and science improve the likelihood that Regenerative Medicine will become clinically routine.

Ex vivo and extracorporeal perfusion with hDAF pig kidneys

The present study was undertaken to determine whether human decay accelerating factor (hDAF) transgene would prevent hyperacute rejection (HAR) while perfused with human blood or extracorporeally in baboons. Four hDAF pig kidneys and three non-hDAF pig kidneys were perfused ex vivo with fresh human blood for 6 h. Additionally four hDAF pig kidneys and four non-hDAF pig kidneys were extracorporeally perfused in baboons and pigs, respectively, for 3 h. In ex vivo perfusion, the color of hDAF pig kidneys remained pink at the end of 6-h perfusion and they had normal histology, while non-hDAF kidneys developed HAR. HDAF pig kidneys had superior function over non-transgenic pig kidneys. Urine output was 17.31 +/- 3.70 ml/h for hDAF pig kidneys, and only 5.81 +/- 0.26 ml/h for non-hDAF kidneys (P < 0.05). Creatinine clearance was 1.16 +/- 1.24 ml/min for hDAF kidneys and 0.22 +/- 0.15 ml/min for non-hDAF kidneys (P < 0.05). Other functional data including potassium, urine specific density, and osmolality were normal in the hDAF kidneys, while in non-hDAF kidneys, serum potassium was elevated to over 9 mmol/l by the end of perfusion (P < 0.01). Non-hDAF kidneys also lost more sodium through urine than hDAF kidneys (173.67 +/- 14.05 mmol/l vs. 109 +/- 31 mmol/l, P < 0.05). In the extracorporeal perfusion, all the baboons tolerated the procedure well with normal hemodynamic and hemotologic profiles. These baboons were well until killed 42 to 56 days after perfusion, although their antiporcine antibodies were greatly elevated. We conclude that hDAF transgene protects against HAR, allowing the pig kidney to function normally while perfused with human blood, and that extracorporeal perfusion using hDAF pig kidneys is a safe procedure in baboons.

Xenoreactive anti-Galalpha(1,3)Gal antibodies prevent porcine endogenous retrovirus infection of human in vivo

The discovery of porcine endogenous retroviruses (PERV) has raised concerns regarding the safety of pig to human xenotransplantation. In this study, we examined PERV infection of human cells in vivo. Furthermore, we examined the effect of human xenoreactive natural antibody on in vivo PERV infection. Human peripheral blood leukocyte reconstituted severe combined immunodeficiency mice were transplanted with porcine aortic endothelial cells (PAEC). PERV gene expression was readily detected in human leukocytes after transplantation. In contrast, human leukocytes harvested from mice treated with human serum or anti-Galalpha(1,3)Gal antibody prevented PERV infection in 6 of 8 mice. These results provide the first evidence that PERV can infect human cells in vivo and that natural xenoreactive antibody can prevent this infection.

Suppressed complement activation in human decay accelerating factor transgenic porcine liver cross-circulated with nonhuman primates

BACKGROUND

We developed an extracorporeal liver perfusion (ECLP) system as a liver-assist device. In this study, we evaluated the safety of the ECLP using human decay accelerating factor (hDAF) transgenic porcine livers in healthy baboons.

METHODS

Livers were isolated from five hDAF transgenic pigs and five nontransgenic pigs for the ECLP. Ten cross-circulations between the ECLP and healthy baboons were performed without immunosuppressive agents. Cross-circulation was discontinued in any of the following circumstances: elevated hepatic arterial (>200 mm Hg) or portal (>60 mm Hg) perfusion pressure, massive exudate from the graft liver, mild macroscopic hemolysis, thrombocytopenia, or 24-hr well-conditioned cross-circulation.

RESULTS

The cross-circulations with nontransgenic porcine livers were discontinued at 4.4+/-1.2 hr (mean+/-standard deviation) because of high perfusion pressure (n=2) or hemolysis (n=3). Three cross-circulations with hDAF transgenic porcine livers were performed for 24 hr; the other two cross-circulations were discontinued at 13 and 17 hr because of massive exudate and thrombocytopenia, respectively. The duration was 20.4+/-5.1 hr. Deposition of membrane attack complex in the hDAF transgenic porcine liver was less than that in the nontransgenic liver, although immunoglobulin-M deposition was comparable. The porcine livers showed no apparent interlobular bleeding or lobular necrosis. All porcine livers maintained bile production during the cross-circulation. No baboons showed any serious complications after the cross-circulation.

CONCLUSION

The hDAF transgenic porcine liver reduced complement activation in xenoperfusion with healthy nonhuman primate blood and led to extended duration of cross-circulation.

Ultra-sensitive and specific detection of porcine endogenous retrovirus (PERV) using a sequence-capture real-time PCR approach

Use of porcine xenografts presents as a possible solution to the current shortage of human allografts limiting transplantation procedures. While no definitive observation of in vivo porcine endogenous retrovirus (PERV) transmission in humans has been reported, the in vitro ability of PERV to infect human cells and the observation of PERV transmission to immunodeficient mice suggest a need for ultra-sensitive techniques to monitor porcine xenograft recipients and contacts for possible PERV transmission. In an effort to enhance current PCR-based PERV detection, the feasibility of combining nucleic acid sequence-capture with use of a quantitative real-time 5' nuclease assay was examined. Sequence-capture by means of oligonucleotide hybridization to a conserved PERV gag sequence and attachment to magnetic beads was used to extract and concentrate PERV A, B and C DNA from sample material containing high levels of background human DNA. Sequence-capture oligonucleotide design incorporated selective substitution of dUTP for dTTP in order to facilitate eventual oligonucleotide destruction. In addition, sequence-capture oligonucleotides were located outside of the amplified region in order to minimize the effects of possible PCR carry-over. Quantitative PCR was then undertaken using a real-time 5' nuclease assay incorporating primers and probe also specific for a conserved PERV gag region. Sequence-capture real-time PCR assessment of PERV levels demonstrated a dynamic range of at least five orders of magnitude, a sensitivity between 0.005 and 0.028 PERV copies per microg background human DNA and a specificity between 98.2 and 100% (95% CI). In contrast, while real-time PERV PCR in the absence of a sequence-capture step demonstrated a similar specificity between 98.4 and 100% (95% CI), the sensitivity of this conventional approach was between 0.2 and 1.0 PERV copies per microg background human DNA. In conclusion, the increased sensitivity of PERV detection obtained by the combined use of PERV-specific sequence-capture and quantitative real-time PERV PCR suggest that this approach should enhance the effectiveness and reliability of monitoring procedures currently applied to porcine xenograft recipients and contacts.

Pig cells that lack the gene for alpha1-3 galactosyltransferase express low levels of the gal antigen

BACKGROUND

The major antigen recognized on pig tissue by primate antibodies is a terminal galalpha1-3gal carbohydrate structure (gal antigen) present on glycolipids and glycoproteins. The production of animals from somatic cells allows for the inactivation of specific genes. It is anticipated that the complete inactivation of the gene encoding alpha1-3 galactosyltransferase, the enzyme that synthesizes the galalpha1-3gal linkage, will result in loss of that antigen from pig organs and tissue and will provide a survival benefit in pig-to-primate xenotransplants.

METHODS

Positive-negative selection was used to produce fetal-pig fibroblasts that were a heterozygous knockout (+/-) of the alpha1-3 galactosyltransferase gene. Nuclear transfer of these cells generated pig embryos and live born pigs with the appropriate genotype. Using a novel selection method with cells from (+/-) embryos, we produced homozygous (-/-) fetal-pig fibroblast cells.

RESULTS

Southern blot analysis of the alpha1-3 galactosyltransferase gene showed that we had produced (+/-) pig embryos, (+/-) live born pigs, and (-/-) pig-fetal fibroblast cells. Fluorescence-activated cell sorter (FACS) analysis with some, but not all, mouse anti-gal monoclonal antibodies and sensitized human serum showed that (-/-) cells still synthesized the gal antigen at 1 to 2% of the level of control heterozygous cells.

CONCLUSIONS

Fetal-pig fibroblasts homozygous for the knockout of the alpha1-3 galactosyltransferase gene appear to express low but detectable levels of the gal antigen.

The in vitro and in vivo effects of anti-galactose antibodies on endothelial cell activation and xenograft rejection

We have previously produced a series of antigalactose (anti-Gal) hybridomas and characterized their heavy chain gene usage. Here we have quantified the affinity of these Abs for the alpha-Gal epitope and characterized their in vitro effects on endothelial cell activation and apoptosis. We report that anti-Gal mAbs derived from Gal(-/-) mice show a range of affinity for the alpha-Gal epitope, and that affinity was generally increased as the V(H) gene usage transitioned from germline sequences to sequences exhibiting somatic maturation. Despite an 85-fold range in affinity, all the anti-Gal mAbs examined induced alpha-Gal-specific endothelial cell activation, and after prolonged exposure induced endothelial cell apoptosis in a complement-independent manner. Only murine anti-Gal mAbs of the IgM or IgG3 subclass, but not IgG1, were effective at initiating complement-dependent cell lysis. Using a novel rat to mouse xenograft model, we examined the in vivo ability of these mAbs to induce xenograft rejection and characterized the rejection using histology and immunohistochemistry. Infusion of complement-fixing IgG3 mAbs resulted in either hyperacute rejection or acute vascular rejection of the xenograft. Surprisingly, infusion of an equal amount of a high affinity anti-Gal IgG1 mAb, that fixed complement poorly also induced a rapid xenograft rejection, which we have labeled very acute rejection. These studies emphasize the importance of in vivo assays, in addition to in vitro assays, in understanding the role of anti-Gal IgG-mediated tissue injury and xenograft rejection.

Cyclosporin monitoring in Australasia: 2002 update of consensus guidelines

Therapeutic drug monitoring of cyclosporin (CsA) has been established as part of the routine clinical treatment of patients following organ transplantation for more than 20 years, and based on contemporary knowledge, many consensus guidelines have been published to assist clinics and laboratories attain optimal strategies for patient care. This article addresses the newer directions in CsA monitoring, with particular reference to the Australasian situation that has evolved since the 1993 Australasian guideline. These changes have included the introduction of alternative assay methodologies, changed CsA formulation from Sandimmun to Neoral throughout Australasia, and alternatives to trough concentration (C0) monitoring, especially 2-hour concentration (C2) monitoring and associated validated dilution protocols to accurately quantitate the higher whole blood CsA concentrations. The revision was prepared following a recent survey of all Australasian CsA-monitoring laboratories where discordant practices were evident.

Analysis of allogeneic versus xenogeneic auxiliary organ perfusion in liver failure reveals superior efficacy of human livers

PURPOSE

To compare the efficacy of allogeneic and xenogeneic extracorporeal liver perfusion (ECLP).

METHODS

An Internet-based keyword search was performed in the established online databases. Univariate and multivariate analysis of variance (general linear method) were performed.

RESULTS

Data from 198 patients were included in the statistical analysis, 142 of whom were treated by ECLP using porcine livers. Baboon livers were used in 29 patients, human livers in 14, and other or mixed species in 13 patients. Pig liver perfusions resulted in a 20% long-term-survival whereas the use of human livers was significantly more successful (survival rate (SVR) 43%, p<0.05). Baboon livers also revealed superior success (41%; p<0.05). Twenty-three patients were treated after 1991, 12 surviving long-term (52%). The latter all belonged to a group of 14 patients who received combined treatment consisting of ECLP and LTx (SVR-86% in this subgroup).

CONCLUSION

Allogeneic ECLP was accompanied by significantly improved outcome compared with discordant xenogeneic ECLP. The role of hyperacute rejection in acute liver failure with reduced complement levels remains controversial. Physiologic disparity between pig and man may be the even more decisive determinant of outcome.

Extracorporeal liver perfusion as hepatic assist in acute liver failure: a review of world experience

BACKGROUND

There are almost no prospective, controlled and randomized clinical trials comparing different approaches towards hepatic assist. In order to create a basis for comparing the value of the existing different hepatic assist methods this article offers a systematic review of the world experience with allogeneic or xenogeneic extracorporeal liver perfusion (ECLP).

METHODS

An Internet-assisted search was conducted in the international literature published from 1964 to 2000. Only articles with a clear description of methodology and outcome of patients were included. For multivariate analysis of variance the general linear method (GLM) procedure was used. Differences within the groups were analyzed by chi-square test. Data of 198 patients were included into the statistical analysis for systematic review.

RESULTS

The long-term survival rate (SVR) of these patients was 26%, thus not exceeding published data concerning SVR under standard intensive care. Age below 40 years (P<0.029), coma stage lower than III-IV (P<0.003), total perfusion time over 10 hours (P<0.024), hepatitis B as cause for acute liver failure (ALF) (P<0.05) as well as use of baboon and human livers (P<0.02) were identified as independent positive prognostic markers for improved survival. ECLP as bridging therapy to liver transplantation was successful in 12 of 14 patients.

CONCLUSION

ECLP using pig livers did not surpass the success of conventional intensive care treatment. An additional effect of transgenic expression of human regulators of complement regulation in porcine livers has not yet been proven. ECLP with human livers not suitable for liver transplantation might prove effective and practicable for temporary hepatic support. Bridging to liver transplantation by long-term ECLP using porcine and human livers appears to have comparable efficacy as bioartificial support methods.

Cyclosporin monitoring in Australasia: survey of laboratory practices in 2000

Therapeutic drug monitoring of cyclosporin has been established as part of the routine clinical management of patients following organ transplantation for some 20 years. The current practices of laboratories in Australia and New Zealand have been surveyed for the year 2000 to update previous similar surveys in the light of considerable changes in CsA formulation (now exclusively Neoral throughout Australasia), assay methods, and blood sampling strategies. The results, representing 93% of CsA laboratories in Australasia (n = 44), found that there was still a plethora of approaches adopted within each organ type for monitoring the established trough (C0) CsA concentration. There was a considerable uptake of 2-hour post-dose sampling (C2) monitoring practices, as demonstrated by assay requests to the responding laboratories, particularly in renal and liver transplantation (46% of centers). Most of these laboratories were also assaying C0 and/or to a much lesser extent, so-called limited sampling strategy AUC (lssAUC) samples at this time. The mFPIA (on TDx and AxSYM analyzers) were still strongly represented (54%) (relatively consistent with international data from the UKQAS proficiency testing scheme at this time) despite on-going concerns expressed in the literature about these methods. However, the Cedia assays had made considerable impact so soon after their introduction with 22% of the laboratories using Cedia or Cedia Plus (Microgenics or Roche Diagnostics; Sydney, Australia) methods. There were a wide variety of dilution protocols adopted in many centers for measuring samples above calibration ranges (such as C2 samples), and hence there was scope for improvement to fully validated techniques. Few centers (16%) made any attempt at interpretation of their results, many seeing their role as purely "measuring" (i.e., an analytical role), not "monitoring" (i.e., including assay and result interpretation). Despite many detailed attempts at providing international or national guidelines for CsA monitoring, etc., there is still considerable scope for improving the quality of the laboratory services offered in this complex as well as expensive area. Several respondents volunteered their support for further Australasian CsA monitoring consensus guidelines.

Comparison of trough, 2-hour, and limited AUC blood sampling for monitoring cyclosporin (Neoral) at day 7 post-renal transplantation and incidence of rejection in the first month

The use of alternative strategies to the traditional pre-dose/trough (C0) blood sampling for cyclosporine (CsA) therapeutic drug monitoring has the potential to revolutionize analytical practices which have, in many centers, been established for some 20 years. While the C0 sample has previously been recommended, current attitudes are increasingly proposing alternatives for assessing CsA exposure, including various limited sampling strategies of the AUC (lssAUC) in the early postdose period, or alternative single-point nontrough samples, such as a 2-hour postdose sample (C2). The present study has reviewed a series of consecutive renal transplant recipients over 18 months where CsA was the primary immunosuppressant. The lssAUC performed at around day 7 posttransplantation included drawing blood at 0, 2, and 4 hours postdose, giving AUC(0-4). The aim of this study was to review the occurrence of acute biopsy-proven rejection in the first month and consider which of (simultaneously measured) C0, C2 or AUC(0-4) was a better early indicator of this adverse outcome. The result was best described by comparing the data from rejectors (n = 13) and nonrejectors (n = 42) for these 3 indices of CsA exposure (i.e., C0, C2 or AUC(0-4)). There was no evidence that C0 predicted the likelihood of such adverse clinical outcomes. In contrast, rejectors tended to have lower mean C2 CsA concentrations, and the incidence of rejection was 0.0 when C2 exceeded 1200 microg/L (n = 10). While the data are limited in the higher C2 CsA concentration range, it is nevertheless consistent with more recent recommendations suggesting that the CsA at C2 should target 1700 microg/L in this first month posttransplantation. As 64% of the patients were also receiving a CsA-sparing agent (diltiazem [DTZ]), the relationships were also investigated to determine whether any affect of concomitant DTZ therapy could be demonstrated. However, in this small sample, no significant affect of DTZ was seen.

Pig endogenous retroviruses and xenotransplantation

Xenotransplantation of porcine organs might provide an unlimited source of donor organs to treat endstage organ failure diseases in humans. However, pigs harbour retroviruses with unknown pathogenic potential as an integral part of their genome. While until recently the risk of interspecies transmission of these porcine endogenous retroviruses (PERV) during xenotransplantation has been thought to be negligible, several reports on infection of human cells in vitro and spread of PERV from transplanted porcine islets in murine model systems have somewhat challenged this view. Here, we compile available data on PERV biology and diagnostics, and discuss the significance of the results with regard to the safety of clinical xenotransplantation.

Development and characterization of anti-Gal B cell receptor transgenic Gal-/- mice

BACKGROUND

The successful clinical application of pig-to-primate xenotransplantation is currently limited by the development of an acute vascular rejection, which is thought to involve an induced humoral immune response to the galactose alpha1,3 galactose (alpha-Gal) antigen. Successful xenotransplantation may require the development of novel methods for removal or neutralization of anti-Gal antibodies and anti-Gal-producing B cells. The large diversity of the B-cell repertoire makes it difficult, however, to isolate and study anti-Gal B-cell development.

METHODS

We have established a transgenic mouse model for investigating anti-Gal B cells by introducing a transgene encoding both heavy and light chains for an anti-Gal IgM antibody into an alpha-galactosyltransferase-deficient (Gal-/-) background. We have characterized the frequency, phenotype, and function of transgenic anti-Gal B cells by multiparameter flow cytometric analysis and ELISA.

RESULTS

ELISA analysis of serum from animals with the transgene in an alpha-galactosyltransferase-deficient background (Tg Gal-/-), from transgenic animals with a heterozygous alpha-galactosyltransferase background (Tg Gal-/+), and from nontransgenic alpha-galactosyltransferase-deficient littermates (Gal-/-) demonstrated elevated expression of anti-Gal antibodies in Tg Gal-/- mice compared with nontransgenic Gal-/- animals and a lack of transgene expression in the Tg Gal-/+ mice. Anti-Gal antibody expression in Tg Gal-/- mice could be increased by immunization with an ovalbumin-Gal glycoconjugate in vivo and through stimulation with lipopolysaccharide in vitro. Multiparameter flow cytometric analysis indicates that 50% to 80% of splenic and peritoneal B cells expressed the transgene and excluded endogenous immunoglobulin gene rearrangements. The majority of these B cells expressed anti-Gal receptors on the surface, as identified by staining with a fluorescein isothiocyanate-bovine serum albumin-Gal glycoconjugate. FACS analysis of the Tg Gal-/- B cells identified them as a population of CD21highCD23lowIgMhigh marginal zone B cells in the spleen and CD5-CD23low B1 cells in the peritoneal cavity.

CONCLUSIONS

These observations suggest that this model can be used to study the regulation of anti-Gal B cells and can establish a reliable source of functional anti-Gal B cells, which could be used to test the effectiveness of alpha-Gal-specific immunosuppressive reagents.

Prolonged function of extracorporeal hDAF transgenic pig livers perfused with human blood

BACKGROUND

The development of genetically modified pigs has renewed interest in the use of porcine liver perfusion in the treatment of acute liver failure.

METHOD

A previously developed model of extracorporeal perfusion has been used to test the function of porcine livers transgenic for human decay accelerating factor when perfused with fresh, whole, human blood. Three experimental groups were studied: alloperfusions (normal pig livers perfused with pig blood) and xenoperfusions of both unmodified and transgenic pig livers with human blood. All livers were perfused for up to 72 hr.

RESULTS

Alloperfusion resulted in the maintenance of good function and histological structure. Stable hemodynamic, synthetic, and metabolic parameters were demonstrated in both unmodified and transgenic liver xenoperfusions; hyperacute rejection was not seen. In both groups, however, the measured parameters of liver function deteriorated toward the end of the 72 hr perfusion period; deterioration was more marked in the nontransgenic group. Xenoperfusions were characterized by a progressive and marked decrease in hematocrit of the circulating blood. Histologically, patchy necrosis was noted in both groups and more retained erythrocytes were seen in the sinusoids of nontransgenic livers, but no other consistent differences were apparent.

CONCLUSIONS

These studies have demonstrated that porcine liver xenoperfusions can be performed for prolonged periods while maintaining good liver function. The use of organs from animals transgenic for a human complement regulator protein confers improvement in some measures of liver function. This preclinical model provides evidence that extracorporeal liver xenoperfusion may be effective in temporary liver support for patients in acute liver failure.

Extracorporeal support of the failing liver

Successes in machine-based extracorporeal support for different organ functions stimulated research in the field of liver support approximately 50 years ago. Initial failure to improve outcome using detoxification methods like dialysis, blood and plasma exchange, or plasmapheresis over sorbents fueled interest in biologic liver support concepts using bioreactors or combined methods. New device configurations, technical improvement of existing detoxification methods, and the refinement in cell culture techniques led to a boost in research on biologic and nonbiologic approaches. Currently, many systems are in the preclinical phase or have entered clinical studies. A number of completed clinical trials have reported a favorable therapeutic impact of the most advanced solutions on the course and outcome of liver failure. Often, findings must be reconfirmed. However, current knowledge suggests that extracorporeal liver support can successfully stabilize liver function, improve the clinical condition of patients, and considerably improve survival in certain subgroups of patients with fulminant hepatic failure and acute decompensation of chronic hepatic failure. Although the initial focus of liver support methods was bridging to liver transplantation, bridging to recovery of organ function and treatment of intractable pruritus are now valuable indications.

Clinical and laboratory evaluation of the safety of a bioartificial liver assist device for potential transmission of porcine endogenous retrovirus

BACKGROUND

The potential risk of transmission of porcine endogenous retroviruses (PERV) from xenogeneic donors into humans has been widely debated. Because we were involved in a phase I/II clinical trial using a bioartificial liver support system (BLSS), we proceeded to evaluate the biosafety of this device.

MATERIALS AND METHODS

The system being evaluated contains primary porcine hepatocytes freshly isolated from pathogen-free, purpose-raised herd. Isolated hepatocytes were installed in the shell, which is separated by a semipermeable membrane (100-kD nominal cutoff) from the lumen through which the patients' whole blood is circulated. Both before and at defined intervals posthemoperfusion, patients' blood was obtained for screening. Additionally, effluent collected from a clinical bioreactor was analyzed. The presence of viral particles was estimated by reverse transcriptase-polymerase chain reaction (RT-PCR) and RT assays. For the detection of pig genomic and mitochondrial DNA, sequence-specific PCR (SS-PCR) was used. Finally, the presence of infectious viral particles in the samples was ascertained by exposure to the PERV-susceptible human cell line HEK-293.

RESULTS

PERV transcripts, RT activity, and infectious PERV particles were not detected in the luminal effluent of a bioreactor. Culture supernatant from untreated control or mitogen-treated porcine hepatocytes (cleared of cellular debris) also failed to infect HEK-293 cell lines. Finally, RT-PCR, SS-PCR, and PERV-specific RT assay detected no PERV infection in the blood samples obtained from five study patients both before and at various times post-hemoperfusion.

CONCLUSION

Although longer patient follow-up is required and mandated to unequivocally establish the biosafety of this device and related bioartificial organ systems, these analyses support the conclusion that when used under standard operational conditions, the BLSS is safe.

Prospects for the temporary treatment of acute liver failure

At present, the most successful treatment of acute liver failure is orthotopic liver transplantation, with survival rates ranging from 70% to 85%. However, mortality rates for liver failure remain high because of the shortage of available donor organs. Therefore, there has been renewed interest in temporary treatment methods for patients with acute liver failure to either allow liver regeneration or await liver transplantation. It is thought that the function of the liver can only be replaced with the biological substrate, e.g. liver cells or a whole liver specimen, which requires the availability of liver tissue from xenogeneic or human sources. In this review, existing temporary liver support techniques are summarized and the potential hazards are described. These include the immunological implications of these techniques, e.g. the host versus graft reaction, which may influence the effectivity of the support system, and in the long run may sensitize the patient to subsequent allogeneic transplantation. The graft versus host reaction is also considered. At present, one of the major concerns is the threat of pig-to-human transmission of activated endogenous retrovirus present in the pig genome. An overview is given of literature concerning the transmission of retrovirus particles in vitro and in vivo. Finally, new solutions for the development of ex vivo systems for temporary treatment of patients with acute liver failure are discussed. These include the use of new immortalized human cell lines and human fetal hepatocytes, and the possibility of isolating, expanding and genetically manipulating stem cells in order to have stable differentiated and committed cells.

Two-hour cyclosporine concentration determination: an appropriate tool to monitor neoral therapy?

Cyclosporine is a critical dose drug for which individualisation by therapeutic drug monitoring is indisputable. Current evidence suggests that a single concentration (C2) taken two hours after cyclosporine administration with the microemulsion formulation better predicts exposure and events than the trough concentration (C(0)), which is routinely used for adjusting the dosage of this drug. Studies have shown that the greatest calcineurin inhibition and the maximum inhibition of IL-2 production occur in the first 1 to 2 hours after dosing. These findings support the concept that the C2 level better reflects immunosuppressive efficacy than the trough concentration. Preliminary data from an outcome study in liver transplant recipients have shown that the incidence of biopsy proven moderate to severe acute rejection was significantly lower in patients managed by C2 monitoring compared with those monitored by C(0). The critical importance of achieving adequate cyclosporine exposure during the first 3 to 5 posttransplant days to prevent acute rejection has been documented in prospective studies with de novo renal and liver transplant recipients. Conversion of maintenance liver and heart transplant patients to C2 monitoring resulted in an amelioration of renal function. Time-dependent target values have been proposed for liver and renal transplant recipients. These require further prospective validation. For routine monitoring of C2 levels on-site validated dilution guidelines are necessary for most of the available immunoassays. C2 monitoring necessitates further organizational requirements which may be judged differently between transplant centers. In particular during the early posttransplant period C2 monitoring is a promising new option to make immunosuppressive therapy with the microemulsion formulation of cyclosporine safer and more efficient.

Efficacy of double filtration plasmapheretic cross-circulation using a high permeability membrane between totally hepatectomized dogs and donor pigs

BACKGROUND

Major obstacles to develop a bioartificial liver are xenogeneic immune reactions and viral infection from donor pigs. To solve these problems, we studied the effect of xenogeneic double filtration plasmapheretic cross-circulation (DFPCC) using a high performance semipermeable membrane on totally hepatectomized dogs.

METHODS

Mongrel dogs, weighing 12-15 kg, underwent total hepatectomy in one stage (n=18). One hr after total hepatectomy, the femoral vein and the jugular vein were cannulated in both dogs and pigs by using the blood access catheter tubes that were connected to the DFPCC system. In the DFPCC circuit, filtrated dog plasma and pig plasma counterflowed in a hollow fiber cartilage at a rate of 25 ml/min for 6 hr and met through a semipermeable membrane with 100 kd nominal molecular weight cut-off (n=5). In control dogs, the circuit was not connected to the pig (n=13).

RESULTS

In vitro mass transfer study suggested that very little immunoglobulins crossed the semipermeable membrane. During and after 6 hr of DFPCC, anhepatic dogs had significantly lower blood ammonia and aromatic amino acid levels than did controls. DFPCC-treated dogs demonstrated decreased intracranial pressure and survived significantly longer than control dogs (20.75+/-3.80 hr vs. 14.75+/-1.30 hr, P<0.05). Histology showed no xenogeneic rejection in both dogs and pigs.

CONCLUSIONS

Our DFPCC systems with a high permeability membrane demonstrated detoxification-function and contributed to intracranial decompression and longer animal survivals without adverse immune reaction and the possibility of zoonosis.

The structure of anti-Gal immunoglobulin genes in naïve and stimulated Gal knockout mice

BACKGROUND

Naturally occurring antibodies (Nabs) that bind to terminal galactose alpha1,3-galactose carbohydrate structures (Gal) are present in humans and Old World monkeys but are negatively regulated in other mammalian species because they express Gal epitopes on their cell surfaces. A Gal knockout mouse (Gal-/-) model, generated by homologous disruption of alpha1,3-galactosyltransferase gene, is capable of producing natural anti-Gal Abs.

METHODS

To study the genetic control of the anti-Gal response, we have generated anti-Gal hybridomas from Gal-/- mice and analyzed VH genes of anti-Gal Abs from naïve animals and from mice stimulated by rat heterotopic heart transplantation.

RESULTS

Six immunoglobulin (Ig)M anti-Gal hybridomas derived from naïve Gal-/- mice exhibited anti-Gal binding activity with some cross-reactivity to related carbohydrate structures. These naïve anti-Gal Abs used five different VH genes in a germline configuration. Anti-Gal IgM hybridomas isolated after a rat heterotopic heart xenograft (4 days) utilized germline VH gene segments from the VH7183 family and exhibited less cross-reactivity. In contrast to mice 4 days after xenograft, we have predominantly isolated IgG anti-Gal hybridomas from mice 21 days after rat heterotopic heart xenografts, indicating an isotype switch. Nine of the IgG anti-Gal hybridomas secreted IgG3 subclass and one produced IgG1. Sequence analysis of the VH gene usage from the induced anti-Gal IgG antibodies demonstrated a restricted gene utilization (VHJ606-V14A).

CONCLUSION

Our results demonstrate that the anti-Gal response in naïve Gal-/- mice is encoded by multiple germline progenitors. In response to a xenograft, the induced anti-Gal Abs exhibited a restricted gene usage and somatic mutations, indicating a positive selection.

Mapping full-length porcine endogenous retroviruses in a large white pig

Xenotransplantation may bridge the widening gap between the shortage of donor organs and the increasing number of patients waiting for transplantation. However, a major safety issue is the potential cross-species transmission of porcine endogenous retroviruses (PERV). This problem could be resolved if it is possible to produce pigs that do not contain replication-competent copies of this virus. In order to determine the feasibility of this, we have determined the number of potentially replication-competent full-length PERV proviruses and obtained data on their integration sites within the porcine genome. We have screened genomic DNA libraries from a Large White pig for potentially intact proviruses. We identified six unique PERV B proviruses that were apparently intact in all three genes, while the majority of isolated proviruses were defective in one or more genes. No intact PERV A proviruses were found in this pig, despite the identification of multiple defective A proviruses. Genotyping of 30 unrelated pigs for these unique proviruses showed a heterogeneous distribution. Two proviruses were uncommon, present in 7 of 30 and 3 of 30 pigs, while three were each present in 24 of 30 pigs, and one was present in 30 of 30 animals examined. Our data indicate that few PERV proviruses in Large White pigs are capable of productive infection and suggest that many could be removed by selective breeding. Further studies are required to determine if all potentially functional proviruses could be removed by breeding or whether gene knockout techniques will be required to remove the residuum.

Functional metabolic characteristics of intact pig livers during prolonged extracorporeal perfusion: potential for a unique biological liver-assist device

BACKGROUND

The clinical development of liver-support devices based on perfusion of either pig hepatocytes cartridges or whole pig livers has been hampered by the ability to use sufficient liver cell mass to provide adequate metabolic support, limited perfusion times, and the potential for patient exposure to pig zoonotic diseases.

METHODS

We designed an original system in which an isolated intact pig liver was perfused extracorporeally under physiological conditions in a closed loop circuit with allogeneic pig blood and constant monitoring of major physiological and functional parameters. The perfusion circuit further included an interface membrane to provide for separation of patient and liver perfusion circulation.

RESULTS

Prolonged (6-21 hr) liver perfusion did not produce significant liver damage as reflected by modest rises in the levels of the serum transaminases, stability of main biochemical parameters (including potassium), and the maintenance of normal cellular morphology. Optimal liver function was documented as measured by lactate consumption, control of glycemia, and the results of clotting studies and functional assays. The perfused liver cleared 82% and 79% of peak bilirubin and ammonia concentrations with clearing kinetics identical throughout perfusion. Indocyanine green clearance was identical to that observed in the living donor before explant surgery.

CONCLUSIONS

In conclusion, the extracorporeal pig liver perfusion apparatus described here allows optimal pig liver function for prolonged periods of time. The microporous membrane to provide separation of donor organ and recipient and the high level of functional activity suggest that this form of liver metabolic support may have important clinical applications.

Analysis of patients treated with living pig tissue for evidence of infection by porcine endogenous retroviruses

The use of pigs as a source of cells and organs for transplantation has the potential to reduce the current chronic shortage of organs for the treatment of many end-stage diseases. The risk of transmission of infectious agents across the species barrier (zoonoses) has to be assessed. Many such agents can be eliminated from the pig herd. However, porcine endogenous retroviruses, which are carried within the pig genome, are not easily eliminated. They can infect primary and immortalized human cells in vitro, but to date no evidence for in vivo infection has been found in retrospective studies of humans exposed to viable porcine cells. Small-scale clinical trials using porcine cells for the treatment of Parkinson's and Huntington's disease are currently in progress. The prospective monitoring of these patients in conjunction with further research into the biology of this virus will help address safety issues.

Lack of cross-species transmission of porcine endogenous retrovirus infection to nonhuman primate recipients of porcine cells, tissues, or organs

BACKGROUND

Nonhuman primates (NHPs) have been widely used in different porcine xenograft procedures inevitably resulting in exposure to porcine endogenous retrovirus (PERV). Surveillance for PERV infection in these NHPs may provide information on the risks of cross-species transmission of PERV, particularly for recipients of vascularized organ xenografts for whom data from human clinical trials is unavailable.

METHODS

We tested 21 Old World and 2 New World primates exposed to a variety of porcine xenografts for evidence of PERV infection. These NHPs included six baboon recipients of pig hearts, six bonnet macaque recipients of transgenic pig skin grafts, and nine rhesus macaque and two capuchin recipients of encapsulated pig islet cells. Serologic screening for PERV antibody was done by a validated Western blot assay, and molecular detection of PERV sequences in peripheral blood mononuclear cells (PBMCs) and plasma was performed using sensitive polymerase chain reaction and reverse transcriptase-polymerase chain reaction assays, respectively. Spleen and lymph node tissues available from six bonnet macaques and three rhesus macaques were also tested for PERV sequences.

RESULTS

All plasma samples were negative for PERV RNA suggesting the absence of viremia in these xenografted animals. Similarly, PERV sequences were not detectable in any PBMC and tissue samples, arguing for the lack of latent infection of these compartments. In addition, all plasma samples were negative for PERV antibodies.

CONCLUSION

These data suggest the absence of PERV infection in all 23 NHPs despite exposure to vascularized porcine organs or tissue xenografts and the use of immunosuppressive therapies in some animals. These findings suggest that PERV is not easily transmitted to these NHP species through these types of xenografts.

Gene targeting in livestock: a preview

Until recently genetically modified livestock could only be generated by pronuclear injection. The discovery that animals can be cloned by nuclear transfer from cultured somatic cells means that it will now be possible to achieve gene targeting in these species. We discuss current developments in NT, the prospects and technical challenges for introducing targeted changes into the germline by this route, and the types of application for which this new technology will be used.